Most synthetic chiral polymers are derived from chiral monomers. Given the limited availability of enantiomerically enriched monomers, it is far more attractive to design synthetic strategies involving the enantioselective polymerization of prochiral monomers. However, reports of such procedures where the polymer chirality arises from asymmetric backbone configurations rather than restricted conformational states (e.g., helicity) are exceedingly rare.
There have been numerous disclosures on the palladium(II) catalyzed alternating copolymerization of .alpha.-olefins with carbon monoxide to form poly(1,4-ketone)s. These .alpha.-olefin-carbon monoxide copolymers, unlike poly(.alpha.-olefin)s, have truly stereogenic centers in the polymer backbone. Using the R,S convention, the dyads can be described as RR or SS, and RS or SR. Similarly, the four possible triads are RRR or SSS, RSR or SRS, RSS or SRR, and RRS or SSR. At the high molecular weight limit, a syndiotactic alternating .alpha.-olefin-carbon monoxide copolymer chain will always show vanishingly small optical activity since the absolute configuration of the stereogenic centers in the backbone alternates (i.e., . . . RSRSRS . . . ). On the other hand, the stereogenic centers in the individual chains of an isotactic alternating .alpha.-olefin-carbon monoxide copolymer sample have the same absolute configurations (i.e., . . . RRRRRR . . . or . . . SSSSSS . . . ), and, thus, the synthesis of optically active, isotactic alternating .alpha.-olefin-carbon monoxide copolymers. should be possible. Furthermore, given the ease with which the carbonyl group can be chemically modified such polymers should be excellent starting materials for other classes of chiral polymers. Brookhart has recently reported the isospecific copolymerization of styrene with carbon monoxide using a chelating bisamine ligand-based catalyst system. (See M. Brookhart et al., J. Amer. Chem. Soc. 1992, 114, 5894). On the other hand, the optimal catalysts for the copolymerization of aliphatic .alpha.-olefins with carbon monoxide are based on chelating bisphosphines. In the few instances in which the synthesis of chiral aliphatic .alpha.-olefin-carbon monoxide copolymers have been reported, the enantioselectivity has either been low or not determined (for example, the possible presence of heterochiral chains has not been eliminated). (See Z. Jiang et al., Macromolecules, 1994, 27, 2694, S. Bronco et al., Macromolecules, 1994, 27, 4436, M. Barasacchi et al., Macromolecules, 1992, 25, 3604, and European Patent Publication No. 384,517).
In a recent paper (Organometallics, 1992, 11, 1766), Consiglio proposed an unusual chain growth mechanism involving cationic Pd-carbene species in order to account for the formation of polymers with spiroketal repeating units under certain conditions. This mechanism differs from the usually accepted two-step mechanism for chain growth involving the alternate insertions of carbon monoxide into Pd-alkyl bonds and olefin into Pd-acyl bonds.
Chien and coworkers (U.S. Pat. No. 5,352,767, in Example 4, for example, and Makromol. Chem. 194, 1993, 2579-2603, at page 2587 product from catalyst 4) show the copolymerization of carbon monoxide and an .alpha.-olefin where the isotacticity of the resulting product is at a maximum of only about 56%.